Powertrain electric control units (ECUs) for vehicles, such as cars, utilize high side transistors (transistors that switch a high voltage side of a load) and low side transistors (transistors that switch a low ground side of a load) to control the inputs to and outputs from the ECUs. High amounts of energy can be dissipated in a high side transistor during a high energy event, such as a short circuit to ground or a battery fault causing a voltage spike. Dissipation of the energy from the high energy event can damage the high side transistor.
One currently used way to protect the high side transistor from such an event is to utilize a sense resistor in series with the high side transistor and monitor the voltage across the sense resistor in real time using a voltage monitoring circuit. When the voltage monitoring circuit detects a voltage in excess of a predefined threshold, the high side transistor is turned off, preventing excess power dissipation within the high side transistor. This technique provides highly accurate results, however the monetary and weight costs associated with utilizing an appropriately sized sense resistor and voltage monitoring circuit are cost prohibitive for some applications, such as vehicle control units.
Another approach used to protect high side transistors from a high energy event involves monitoring a drain to source node voltage of the high side transistor. In this approach, when the drain to source node voltage exceeds a predetermined threshold, with the high side transistor is saturated, the monitor detects that a high energy event is occurring and appropriate action is taken to protect the transistor. Due to the nature of MOSFET type transistors, however, there is a time period after receiving a control signal turning the transistor on and before the transistor is fully on or saturated. Within that time period the voltage across the drain to source nodes of the transistor steadily declines to zero volts, and during the switching on time period a high energy event is not detectable across the drain to source modes of the transistor. As a result, protection circuits cannot respond to high energy events occurring during the switching time period, and the high side transistor can be damaged.